Aneth Oil Field, Giant Carbonate Stratigraphic Trap, Paradox Basin, Utah, U.S.A.

Abstract

The Aneth oil field (approximately 400 million bbl original primary reserves) occurs within an isolated southeast-northwest-trending Pennsylvanian (Desmoinesian) algal carbonate bank buildup located approximately 25 mi (40 km) basinward of the main carbonate shelf of the Paradox basin in Utah. Desmoinesian deposition was strongly cyclic, with as many as 30 main evaporite-carbonate cycles recognized. The Aneth algal bank is part of the Desert Creek cycle, which is the first cycle above that of the most widespread salt-bearing cycle. The Desert Creek carbonate facies grades basinward to increasingly prevalent evaporite facies, dominated by thick salt beds in the basin center. Within the isolated algal-mound facies, the Desert Creek cycle is characterized by a vertical succession that includes (1) basal black, organic-rich, dolomitic, silty shale, (2) overlying dark chalky, finely crystalline dolomite, which grades upward into (3) the main carbonate mound facies, dominated by accumulations of calcareous, leafy green algae with associated brachiopods, fusulinids, small foraminifera, ostracodes, and rare corals. Mound buildups are overlain by a relatively thin, black, organic-rich shale unit, the basal bed of the succeeding mound-bearing Ismay cycle. This unit thickens markedly on the flanks of the Desert Creek and other mound buildup. Interrelated depositional processes of these variousmore » sedimentary facies have resulted in a close association between source rock, large reservoir, and seal at Aneth, and contributed significantly toward the large size of the Aneth accumulation.« less